1. Field of the Invention
This invention relates to a porous isotropic carbon-carbon composite having a carbon fiber as a reinforcement material, and a process for producing such a porous isotropic carbon-carbon composite. More particularly, it relates to a porous isotropic carbon-carbon composite having a suitable porosity, a substantially isotropic mechanical strength, and superior properties in resistance to heat, chemicals and abrasion, etc., and a process for producing the same.
A carbon-carbon composite produced according to the process of the present invention is useful as a filter element material, a catalyst carrier, an electrode for fuel batteries, a bearing part, a dry cell part, and a storage battery part, etc.
2. Prior Art
It is known that a porous isotropic carbon-carbon composite can be obtained by effervescing impregnation of a liquid carbonizable material into a reinforcement material of a high strength high modulus carbon fiber while the carbon fiber is wound to a desired shape, or into a material of a structure having a high strength, high modulus carbon fiber as its principal material, e.g. woven fabrics, three-dimensional woven fabrics, non-woven fabrics, unidirectionally oriented sheets, or by cutting a reinforcing material of a high strength high modulus carbon fiber short, mixing a liquid carbonizable material therewith and causing it to foam, and carbonizing the liquid carbonizable material that is a precursor of carbon matrix under inert atmosphere and, if necessary, graphitizing the carbonized product.
The problem of this method is difficulty of isotropical orientation of the reinforcement material. Thus, the strength of the composite material differs dramatically direction to direction, and in many cases, products of the composite material have a weak point, and it is necessary to handle them as a fragile thing. Also, distribution and orientation of pores tend to be ununiform. This is considered as one of the reasons why the strength of such a composite material differs direction to direction.
In order to solve this problem, it has been carried out in prior art method to cut a reinforcing fiber very short or to make the proportion of the content of a reinforcement fiber smaller. However, there is a problem in this method that the effectiveness of the use of this reinforcement material is not sufficient. As an alternative solution, a method is proposed in which very fine glass hollow cubes are mixed. However, this leads to introduction of impurities into a composite after carbonization, as well as reduction of electric properties and corrosion resistance. Also, there is a problem in this method that production of a continuous pore type porous composite is difficult.
As a process for producing porous carbon products which is different from the above-mentioned process, there is disclosed in U.S. Pat. No. 3,829,327 a method in which a web of a carbon fiber is coated with pyrolytic carbon formed by CVD process. This method has an advantage of forming products superior in chemical and electrical properties, but there are problems that this method is costly and strength is notably reduced if a carbon material having a large porosity is made.
Also, in U.S. Pat. No. 3,991,169, a process wherein a higher alcohol is caused to adhere to a pitch fiber mat to help local melt-adhesion of a pitch fiber is disclosed. But this method has a problem that a product having a high mechanical strength is difficult.
Also, U.S. Pat. No. 3,960,601 discloses a fibrous web obtained by infusiblization and carbonization treatment of a pitch fiber web produced by blow-spinning. However, a porous carbon material produced by this method is essentially an aggregate of a fiber and pore size is small. If it is attempted to make the porosity larger, there is a problem that its mechanical strength is reduced.
It is an object of the present invention to solve the problem of difficulty of handling during molding and carbonization of a porous carbon-carbon composite, which difficulty is caused by notable weakness in properties such as mechanical strength in a certain direction due to notable directional properties (anisotropy) of the reinforcement material.
The above-mentioned object can be accomplished by the process of the present invention.